V-O-Ag Linkages in VAgOx Mixed Oxides for the Selective Oxidation of p-Xylene to p-Methyl Benzaldehyde

Selective oxidation of alkanes/aromatics to desirable oxygenated molecules offers a pathway toward an efficient chemical process. However, high selectivity is hampered due to the overoxidation reaction. Here, we show that alpha-Al2O3-supported VAgOx mixed oxides can selectively catalyze the oxidation of p-xylene to p-methyl benzaldehyde with an up to 92% selectivity, as compared to 57% on VOx/alpha-Al2O3, at a 1% conversion and 295 degrees C. Even at a higher conversion (similar to 6%) via increasing the residual time, the aimed selectivity on the optimized mixed oxides is about 2 times that of their counterpart, i.e., 73% vs 35%. A combination of kinetic investigation and temperature-programmed surface reaction experiments indicates that the enhanced selectivity on VAgOx mixed oxides is attributed to the terminated parallel combustion reaction (forming CO2) via weakening the adsorption strength between aromatics and the catalyst. Furthermore, the linear correlation between the Raman intensity and the formation rate of products, obtained in operando Raman experiments, suggests that the V-O-Ag linkages in VAgOx/alpha-Al2O3 are active sites for selective oxidation of p-xylene, as further verified by their complete regeneration under an O-2 atmosphere after consumed by p-xylene. This work provides spectroscopic evidence for understanding active sites on mixed oxide catalysts and supports the importance of modulating the chemistry of lattice oxygen for selective oxidation reactions.